When you think about it, Nintendo's done a sequel/prequel to every Zelda game except Twilight Princess and, so far, Skyward Sword. That they would deviate from this formula for so long is a bit strange.
Zelda Wii U could, of course, be the next installment in any one line of Zelda games - A Link to the Past is receiving its own direct sequel on the 3DS, proving Nintendo can go back and revive older Zelda games. I don't think Twilight Princess particularly "needs" a sequel/prequel, but it would rather nicely wrap up or polish some elements, such as:
#1: The supporting cast. Wasn't Ilia supposed to be Link's love interest? Was it really so significant that Colin overcame his one vague quote of not liking swords and took up a weapon? Who took over Malo Mart when Malo went back home? (Chudley?) All these characters appeared and acted important at the time, but then little came of them - unfortunately, a recurring problem in Zelda. A sequel could give these characters the spotlight they deserve.
#2: Why the Mirror of Twilight is still intact. Spoiler alert, the ending of Twilight Princess makes traveling to the Twilight Realm kind of impossible, yet the Mirror of Twilight is apparently still there as the Dark Mirror in Four Swords Adventures, chronologically the next game to take place on that timeline. 'Splain, Nintendo.
#3: Link's wolf form. Novel as the idea was, it wasn't fleshed out much in Twilight Princess. Human Link gets all kinds of new techniques and weapons, yet Wolf Link is clearly reduced to a weaker and occasionally necessary sideshow. I'm not saying we need to see Wolf Link again, but it would be kind of nice to see a Wolf Link on par with his human side, eh?
Not to mention I enjoy Twilight Princess's darker atmosphere anyway, so it'd be nice to return. Whether Zelda Wii U will be that sequel/prequel is hard to say, but E3 is right around the bend. I hope we find out soon.